The present invention relates to a control system for an automatic transmission of a motor vehicle having an antilock brake system, and more particularly to a system for controlling the transmission when the antilock brake system is in operation.
The antilock brake system is installed on a brake system for wheels of the vehicle. The antilock brake system is provided for preventing the wheels from completely locking at rapid braking or by braking on slippery surfaces such as a snowy road, thereby ensuring directional stability and steering control of the vehicle during the braking operation and improving safety of the vehicle.
However, if the engine braking is effected in the antilock operation, the engine braking force is exerted on the wheels to lock them in spite of the antilock operation. Since the antilock brake system operates to reduce the hydraulic pressure in the system in accordance with the deceleration of the wheel speed, if the wheel speed is disturbed by the engine braking, the efficiency of the antilock brake system is reduced. Thus, it is desired to eliminate the disturbance due to the operation of the driving system, for providing a proper antilock brake system control.
On the other hand, an automatic transmission which is constructed to effect the engine braking in particular ranges in accordance with a position of a shift lever or a selected range is used. Accordingly, it is necessary to control the engine braking operation in the automatic transmission in accordance with the operation of the antilock brake system.
Japanese Patent Application Laid-Open 60-135356 discloses a control system in which the operational range of an automatic transmission is changed to a neutral range when an antilock brake system operates. Namely the power transmitting system from the engine to wheels is disconnected when the antilock brake system is operated. The disconnection of the transmitting system, where the engine braking is not effected, continues as long as the antilock brake system operates. Accordingly, in the case that the coefficient of friction between the tires of the vehicle and the road is small, the vehicle is not effectively braked, resulting in an increase of the stopping distance of the vehicle.